Auxiliary drive system with neutral

ABSTRACT

The steerable wheels of a hydraulically driven vehicle are provided with auxiliary motors which are connected in parallel both with each other and with the motor which powers the main drive wheels. A selector valve permits the motors for the steerable wheels to be optionally connected and disconnected from the reversible, variable displacement main pump. Flow dividers in the hydraulic lines limit the maximum flow of fluid to any one hydraulic motor. The auxiliary motors are of the variable displacement type, enabling them to be destroked to neutral when disconnected from the main pump. Destroking to neutral eliminates energy loss due to the pumping of large quantities of hydraulic fluid through a closed loop when the motors are driven by the wheels.

BACKGROUND OF THE INVENTION

This invention relates generally to hydraulic drive systems and morespecifically, to a hydraulically driven vehicle wherein motors on two ofthe wheels can be optionally powered as needed. Generally, anoff-highway vehicle includes a load carrying body supported on both apair of main traction wheels and a pair of steerable wheels. Whenoperating under heavy load in soft soil conditions, it is desirable tohave the steerable wheels powered as well as the main traction wheels.Once the loaded vehicle reaches a hard surfaced road, however, it isbetter to be able to deactivate the hydraulic drive from the steerablewheels and rely on the main drive wheels for propulsion. With myinvention this is possible.

Various hydraulically driven vehicles are known. The U.S. Pat. toJennings, et al No. (3,736,732) shows an auxiliary drive system fordriving the steerable rear wheels of a combine. U.S. Pat. No. 3,997,017discloses an auxiliary hydrostatic front wheel drive system having fluidpressure actuated clutches to effect a driving combination between themotors and wheels. U.S. Pat. No. 4,072,009 presents the pressure systemfor driving the wheels of a skid steered front loader. U.S. Pat. No.4,140,196 shows an auxiliary drive system for optionally powering thesteerable wheels of a four-wheel vehicle.

None of the above disclosed systems have the inherent advantages of myinvention. A vehicle equipped with my invention does not require theoperator to dismount and reposition a wheel-hub mechanism for eitherengagement or disengagement. More importantly, when the steerable wheelsof my invention are operating in the unpowered mode they do not presenta drag of several horsepower on the vehicle. This is because theauxiliary motors used in my system do not pump large quantities of fluidaround a closed loop when the wheels turn the motors causing them to actas pumps.

SUMMARY OF THE INVENTION

It is an object of my invention to provide a highly efficient auxiliarydrive system for a vehicle. Auxiliary power is supplied to the twosteerable wheels of a vehicle by a pair of hydrostatic motors which aresupplied fluid from the variable displacement pump which drives the maintraction wheels. Destroking type hydrostatic assist motors are used.

When auxiliary power is not required, the supply of hydrostatic fluidcan be disconnected by a selector valve. The selector valve performs twotypes of control functions. First, it controls the delivery of lowpressure line charging fluid to the auxiliary motors. Second, theselector valve serves to connect or disconnect the auxiliary motors withthe hydraulic line system connecting them with the variable displacementpump that supplies high pressure hydrostatic fluid. When the selectorvalve is moved to its "disconnect auxiliary drive" position, chargingpressure flows through the selector valve and on reaching the auxiliarymotors, causes them to be destroked to a neutral status. At the sametime the supply of high pressure fluid from the variable displacementpump is cut-off from the auxiliary motors and the motor input line shortcircuited to the output line.

There are flow dividers in the high pressure lines which supply fluid tothe auxiliary motors. The flow dividers allow the output of the variabledisplacement pump to drive all motors in parallel. No one motor canreceive an excessive flow of fluid during a wheel spin-out. The flowdividers have been selected to allow a 10 percent variation in fluidflow to each auxiliary motor. This amount of variation has beendetermined as necessary to accommodate the differential actionassociated with steering.

With my invention, auxiliary power is provided on an operator selectablebasis at each steerable wheel. This greatly improves vehiclemaneuverability in soft soil conditions. The assist circuit is fullyprotected from uncontrolled spin-out conditions which could damage thehydrostatic motors and detract from vehicle performance. Activating asingle selector valve disconnects the assist circuit completely from themain traction system and destrokes the assist motors to zerodisplacement. This complete auxiliary drive circuit provides afunctional power assist for any four wheel drive vehicle. The controlsare simple and reliable. The auxiliary motors can be destroked to zerodisplacement so as to eliminate any induced flow, pressure drop or heatbuildup in the auxiliary components when the vehicle is being towed oroperated at a high velocity with the two wheel drive system. Thiseliminates hydraulic energy loss which significantly reduces operationalheat buildup in the circuit and therefore extends the life of thecomponents and greatly improves overall circuit efficiency.

BRIEF DESCRIPTION OF THE DRAWING

The sole FIGURE of the drawing is a diagrammatic illustration of thepreferred embodiment of a hydrostatic system for driving the steerablewheels of a vehicle.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The lone drawing FIGURE shows the general arrangement of both the mainand auxiliarly drive system. Hydraulic energy from a variabledisplacement pump 10 powers the entire system. The main drive wheels 12of the vehicle are driven through a conventional differential gear box14. Gear box 14 may also contain variable ratio reduction gearing sothat it can be properly driven by reversible hydraulic motor 16. Motor16 is reversibly driven through hydraulic lines 18 and 20 by variabledisplacement pump 10. Pump 10 is conventionally driven by an internalcombustion engine (not shown).

It will be understood that motor 16 has units coupled thereto whichprotect the system against excessive pressure. These units include: twohigh pressure relief valves, one at each side of the motor; a shuttlevalve and a low pressure relief valve. Inclusion of these safetyfeatures protect against seal and line rupture.

The hydraulic lines of the main drive system are maintained in a chargedcondition by a low pressure, low volume charging pump 22 which drawsfluid out of sump 23 via line 25. In most implementations, pump 22 isdriven by the same prime mover which powers pump 10. Charging pump 22connects to hydraulic lines 18 and 20 via line 24 and check valves 26and 28. The pressure level established at the output of pump 22 ismaintained by pressure relief valve 27. Excess oil is returned by valve27 to sump 23, thereby keeping charging pressure in line 24 to a presetlevel. The charging circuit is thus able to make up for any leakage inthe main or auxiliary drive system.

Connected in parallel with the main hydraulic motor 16 are a pair ofauxiliary motors 30 and 32. The auxiliary motors are powered fromvariable displacement pump 10 by means of hydraulic lines 34 and 36.Control of the fluid in lines 34 and 36 is achieved by selector valve38. In the drawing, selector valve 38 is shown in the disengagedposition wherein only hydraulic motor 16 would be driven. Auxiliarymotors 30 and 32 are unpowered since fluid in lines 34 and 36 cannotflow through spool 42 of selector valve 38. In the position shown in thedrawing, charging pressure from pump 22 flows through charging line 41,through both spools 40 and 42, and from spool 40 thence via line 43 tothe head ends of control pistons 44 and 45 of auxiliary motors 30 and32. The charging pressure passing through spool 42 flows sequentiallythrough line 48, first flow divider 51, and then in parallel throughlines 53 and 55, first shuttle valve 62, second shuttle valve 63 andthence to the rod ends of control pistons 44 and 45. Charge pressure isthus exposed to both ends of the control pistons 44 and 45. Equalpressure acting on the differential areas of the control pistons permitssprings (not shown) to destroke the auxiliary motors.

Actuation of spool 40 in selector valve 38 (This was a solenoid switchin the unit reduced to practice) causes charging pressure to be appliedto the end of spool 42 and at the same time releases charging pressurefrom the head ends of control pistons 44 and 45. Pistons 44 and 45 havetheir rod ends attached to the stroking mechanisms of motors 60 and 61respectively. Application of pressure to the end of spool 42 depressesspring 46 and with the endwise movement of the spool mechanism connectsline 34 with line 48 and line 36 with line 50.

Forward pressure in line 48 causes fluid to flow through first flowdivider 51. Flow divider 51 is of conventional design in that itcontains a pair of fixed orifices which serve to equalize fluid flowalong output lines 53 and 55. Similarly, reverse pressure in line 50draws fluid back from auxiliary motors 30 and 32 via lines 54 and 56,through second fluid divider 52, along lines 50 and 36, and into pump10.

Auxiliary motors 30 and 32 are of the destroking type. As shown in thedrawing auxiliary motor 30 includes a first destroking motor 60, a firstshuttle valve 62 and a first control piston 44. Similarly, auxiliarymotor 32 includes second destroking motor 61, second shuttle valve 63and second control piston 45. Destroking motor 60 drives wheel assembly64 through a planetary gear reduction hub (not shown). In a similarmanner, destroking motor 61 drives wheel assembly 65 through a secondplanetary gear reduction hub incorporated therein.

Forward hydrostatic pressure in line 48 passes through flow divider 51and into lines 53 and 55. Forward pressure in lines 53 and 55 actuatesshuttle valves 62 and 63 to the positions shown in the drawing. Thispositioning of the shuttle valves results in directing work pressurefluid to the rod ends of control pistons 44 and 45. Pressure on the rodends of control pistons 44 and 45 places the destroking motors 60 and 61in their maximum displacement positions. As a result, forward pressurehydrostatic fluid flowing along lines 53 and 55 starts motors 60 and 61turning in a forward direction. Oil returns from destroking motors 60and 61 along lines 54 and 56, through flow divider valve 52, line 50,the return orifice through spool 42 of selector valve 38 and then alongline 36 to the input port of variable displacement pump 10.

When the flow of fluid through pump 10 is reversed the direction oftravel of all motors change. The main drive motor 16 reverses directioncausing the vehicle to begin to move backward. With selector valve 38 inits actuated position, spring 46 will be depressed, allowing highpressure fluid to flow along line 50, through second fluid divider 52and into line 54 and 56. Pressure in lines 54 and 56 causes shuttlevalves 62 and 63 to switch positions, meaning that they assume theopposite state to that shown in the drawing. With the switching inposition of shuttle valves 62 and 63, the pressure in lines 54 and 56actuates control pistons 44 and 45 to maintain motors 60 and 61 in theirmaximum displacement positions. Flow of reverse pressure fluid from line54 through motor 60 and out line 53 causes wheel assembly 64 to turn ina reverse direction. Similarly, flow of reverse pressure fluid from line56, through motor 61 and out line 55 causes wheel assembly 65 to turn ina reverse direction. Second fluid divider 52 prevents either motor 60 or61 from receiving an excess amount of fluid if either wheel 64 or 65starts slipping.

Lines 66 and 67 connect motors 60 and 61 with sumps 68 and 69,respectively. This provides a return path for motor leakage. In actualpractice sumps 68 and 69 will be in communication with sump 23 to assurethat all working fluid remains in the system.

To deactivate the auxiliary system, spool 40 of selector valve 38 isreturned to the position shown in the drawing. In the unit reduced topractice, spool 40 was part of a solenoid type switch and therefore,de-energizing of the solenoid accomplished the task. Spool 40 could alsobe a manually controlled unit. Once spool 40 returns to the positionshown in the drawing, fluid pressure on the end of spool 42 drops andspring 46 returns spool 42 to the position shown. This interrupts thesupply of high pressure fluid coming from pump 10. Further, the orificeswithin spool 42 are configured such that lines 48 and 50 are bothconnected to the charging pressure of line 41. This assures that no airpockets appear in the lines during the deactivated period.

Return of spool 40 to the position shown in the drawing connects thecharging pressure in line 41 with line 43. Charging pressure in line 43actuates control pistons 44 and 45, thus destroking motors 60 and 61 totheir minimum displacement position. With both hydraulic motors 60 and61 in their minimum displacement conditions, the vehicle can be operatedat high speed in two wheel drive without having the assist motorspumping large volumes of oil around a closed loop thereby causing a dragof many horsepower on the system. Tests on prior art systems showedclosed loop power losses of at least 15 Hp. With a system incorporatingmy invention, closed loop pumping losses are non-existent because motordisplacement is zero.

In addition to supplying make-up oil to the auxiliary motor assembly,the continued application of charge pressure to the rotating componentsof motors 60 and 61 serves to hold these components against theirrespective motor swash plates, thereby preventing motor damage duringthe period that wheels 64 and 65 turn the motors.

While only a single embodiment of the invention has been presented,various modifications will be apparent to those skilled in the art.Therefore, the invention should not be limited to the specificillustration disclosed, but only by the following claims.

I claim:
 1. An auxiliary drive system for a vehicle having a variabledisplacement main pump, a sump for providing a reservoir of hydrostaticfluid, a charging pump for drawing fluids from the sump via ahydrostatic line and delivering said fluids via interconnecting fluidlines to maintain all system components in a fully charged condition,the auxiliary drive system comprising:a pair of variable displacementtype reversible hydraulic auxiliary motors, each of said motors beingoperably and individually connected to a wheel assembly; a pair ofcontrol pistons for varying the displacement status of said auxiliarymotors, each one of said pistons having its rod end mounted forreceiving fluid pressure to vary the displacement of each of saidauxiliary motors; first fluid line means connecting the two sides ofeach of the auxiliary motors with the variable displacement main pump,said fluid lines being arranged so that the auxiliary motors areconnected in parallel with each other; selector valve means havingalternate on and off positions for controlling the flow of hydrostaticfluid in said first fluid lines, thereby allowing the auxiliary drivesystem to be activated or deactivated at will; a pair of shuttle valves,one in close proximity to each of said variable displacement typereversible motors, each of said shuttle valves being encirculated tomaintain said motor in its maximum displacement condition during bothforward and reverse actuated states; pressure compensated flow dividersinterposed in said first fluid line means for limiting the maximum flowof hydrostatic fluid to any one auxiliary motor; second fluid line meanshaving one end in communication with the output of said charging pump,the second end of said second fluid line being connected to supplycharging pressure to said auxiliary motors; and said selector valvemeans including valve means for controlling fluid flow to said controlpistons enabling said pistons to be actuated to either of two states,one state being such that the auxiliary motors are maintained in theirmaximum displacement condition when said motors are being driven byoutput of said variable displacement pump, the second state being suchthat the auxiliary motors are maintained in their minimum displacementcondition when said auxiliary motors are in the deactivated condition.2. The invention as defined in claim 1 wherein the selector valve meansincludes in its "on" position circuitry connecting said first fluid linemeans with both sides of said variable displacement pump thereby placingopposite sides of each of said auxiliary motors in communication withsaid variable displacement pump, said selector valve means in its "on"position simultaneously allowing charging pressure to drain from saidcontrol pistons into said sump through said second fluid lines.
 3. Theinvention as defined in claim 1 wherein the selector valve meansincludes in its "off" position circuitry which terminates all variabledisplacement pump fluid flow to said auxiliary motors, said selectorvalve means simultaneously allowing charging pressure fluid to maintainthe auxiliary motors in their minimum displacement condition.
 4. Theinvention as defined in claim 1 wherein the wheels connected to each ofsaid auxiliary motors are steerable.
 5. The invention as defined inclaim 1 wherein two pressure compensated fluid flow dividers areutilized.
 6. The invention as defined in claim 1 including use of asolenoid actuated type switch in said selector valve means.